Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore into the subterranean formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe (e.g., casing) is run in the wellbore. The drilling fluid is then usually circulated downward through the interior of the pipe and upward through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. Next, cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. During the life of a well, from drilling to exhaustion, understanding the evolution of the materials contained in the annulus provides important information on stability within a wellbore. A variety of sensors, temperature, pressure, pH, etc. are routinely placed around or in the casing string all intended to provide information that can be interpreted and translated into a picture of what is taking place downhole.
Prior attempts to monitor annulus composition have been complex and costly. One method that has been used during drilling is to monitor changes in the composition of the drilling mud. Since the drilling mud picks up drill cuttings from the rock formation it is possible to ascertain changes in the rock formation from the changing composition of the drilling mud. The method includes an acoustic sensor using a Doppler technique. In this technique, the speed of sound of the fluid inside the casing string is measured and the speed of sound of the fluid in the annulus is measured. By comparing the two measurements, the fluid composition in the annulus may be is determined.
A better understanding of the various disclosed system and method embodiments can be obtained when the following detailed description is considered in conjunction with the drawings.